1. What datapack(s) do you have installed (including ones made by me- include version if possible).

Alll datapacks are installed via the Modrinth app. Filter is set to vanilla and 1.20.4.

ALL-Recycling-Recipes.zip
AutoTree.zip
Bell_Finds_All_Raiders.zip
Chest Village[1.20.3-1.20.4].zip
Code of Copper-v0.3.1.zip
Copper Wrench.zip
Effortless Harvest 1.20.2 1.2.0.zip
Elevators 1.20.4.zip
eroxenwire-v2.1.0-datapack.zip
fishingsim.zip
Gamingbarn's Guns - Resources V1.21.1.zip
gm4_auto_crafting_1_20.zip
HammerEnchants_1.20-1.20.1_v1.2.3.zip
i_wanna_travel-datapack-0.1.zip
Kaths-Teleporter.zip
kogtyv-WOM-1.2-Datapack-1.20.4.zip
Krougal cauldron concrete v2.0.9 (MC 1.20-1.20.4).zip
MechanizationConfig_v4.2.3.zip
MechanizationDatapack_v4.2.3.zip
Minions v1.1.2.zip
Multipack Caves.zip
No End Void.zip
Philip's Ruins1.20x-2.1.zip
Player Graves.zip
Structures-Addition.zip
structures-rebalanced-advancements-v1.0.0.zip
StructuresPack v1.0.0.zip
technical-enchant-v7-4-2_1.20.4.zip
vanillastructures-1.20.x-1.3.1.zip
VoidTP.zip
WASD YouTuber Swords V1.8.5 MC_1.20.4.zip
[1.20.2+] Enhanced Allays V3.2.zip
[1.20.4]-bubblevehicles-3.3c.zip

2. Ensure the datapack is loaded with /datapack list

Had to open LAN World for this to work (cheats = 0n). Yes. Both config and datapack of mechanization are loaded.

3. What environment are you running the datapack? (Vanilla single player, Vanilla multiplayer, Forge, fabric, spigot, bukkit, paper, optifine, realms, etc). Please specify exact Minecraft version as well.

Vanilla Minecraft 1.20.4 without any modloader (Forge, Fabric, Quilt, NeoForge - NOTHING)
I'm very sorry to report that I did upgrade to Win 11. I did that to be able to access some 4TB harddrives fully.

8. What if happening vs. what you expect to happen?

In the first world that I hat with Mechanization the manual worked just fine. But the two Welcoming pages were missing, what I didn't realize at that moment. The book startet with its 9 icons and clicking on one of them closed the book but next time opening it I was in the chapter I wanted.

Then I installed some more structure datapacks and began a new world to see the changes. Everything works fine.
Now when I opened the manual I see a greeting page, which is new for me. Then another page and then my 9 icons - yay.
Pressing an icon closes the book and sets a value to -1 for first icon. (Chat message)

Now when I open the book again I am NOT in the chosen chapter. I'm back on the greetings page. Repeating this with any icon always takes me back to the intro. No more book contents for me.

Sounds like there's a conflicting datapack? If it's working without extra datapacks, I would try adding a couple datapacks at a time until it breaks to narrow down which one is causing the problem.

Also, before that you can check the console for errors (launcher -> settings -> open output log when Minecraft starts)

Difference between the two worlds:

First world: Generated and first visited without any datapack. Then the datapacks were added and the game was ended and started multiple times. Most datapacks just worked fine.

Second world: The datapacks were added on world generation with the datapack button in the world gen GUI. Some datapacks have problems.

New profile with only mecha and config datapack and mecha resourcepack loaded (ok, and faithful 32).
The datapacks were inserted on the world creation datapack GUI.
Book is given when entering world.
First two greeting pages appear on first use.
Then the nine icons.
Selecting an icon closes the book and sends a message about a set value to the chat.
Opening the book again shows the 9 icons. Only possible action is selecting an icon and close the book.
The error is now the same as in my OP.

Just to proof that it works I started another world without any datapack. Closed the world and added datapack after world generation. Next visist the book is given.

Damn! Now it also not works. The thought was that it now would work because of reasons. But it is now the same behavior.
The nine icons give a message and close the book. Next time opening the book it are again the nine icons.

Thinking hard what I did to make it work on the very first time I remember that I didn't have neither config nor resourcepack as I installed it from the Modrinth app. I did get a book but this was not readable. The book only showed some hieroglyphs. I got the missing files from the latest distribution from here. Over multiple world restarts I worked my way to a correctly installed resourcepack.

Then the book was readable. An icon put a message and closed the book. Next time the chosen chapter was open.